The present invention relates to a new and improved method for reforming or rejuvenating a used tubular mold employed for continuous casting, wherein damaged locations at the inner surfaces of the mold are ground.
For the continuous casting of high melting metals, such as typically steel and so forth, there are employed, among other things, independently of the strand cross-section, one-part tubular molds or known constructions. These molds can be fabricated from copper or copper alloys.
According to a presently known fabrication technique, as taught in German Pat. No. 1,809,633, an arbour or mandril is forced into a pre-formed tubular mold blank. This arbour or mandril possesses the final internal dimensions and the final shape of the tubular mold. Thereafter, the tubular mold is plastically deformed by a drawing operation in conformity with the shape of the arbour or mandril. Therafter, the mandril is again removed from the tubular mold which has solidified during the cold working.
In most instances the inner surfaces of continuous casting molds are coated with a material of greater wear resistance than copper in order to prevent the deposition of copper upon the cast product and to increase the longevity. A frequently employed coating process is the generally known galvanic chrome-plating. For this purpose an anode is placed into the copper tubular mold serving as the cathode, and the anode and cathode are immersed in a chromium acid solution. Following application of a direct-current voltage disassociated chromium ions are separated out on the copper. The quality of the separated-out chromium layer, depends, among other things, upon the actual construction or configuration of the continuous casting mold at the corner regions, but also upon the surface properties of the material which is to be coated. A rough surface also will be rough even after the chrome-plating operation. In terms of a spent or used mold this means that because of the roughness which prevails by virtue of the post-machining, for instance the grinding work, there will be present within the continuous casting mold an increased friction of the continuously cast strand within the casting mold, so that there can exist increased wear and transverse fissures in the cast product which, in the worst case, can lead to metal break-out.
According to a further method which has become known to the art and which is utilized more recently, for instance as taught in German Pat. No. 2,533,528 and the corresponding U.S. Pat. No. 4,220,027, used, worn, straight or curved tubular molds which also possess a conical hollow mold compartment, can be recalibrated to the original mold dimensions by an explosive forming technique. With this method the inner surfaces of the tubular mold again have imparted thereto the properties of a new mold. In practicing such method, prior to the explosive forming operation, the surface flaws or defects are manually ground away with the aid of a suitable grinding apparatus, in order to eliminate right from the start material overlaps of the faulty edges during the explosive forming operation.
If such type of recalibrated tubular molds are coated, then it can happen that the previous material protruberances or raised portions which were formed due to the grinding operation and which have been pressed flat because of the explosive forming operation--a scratch viewed in enlargment appears as an extended crater composed of a depression within the material and a lateral mound or pile of displaced away material--will again reform as raised portions or protruberances due to the action of the electromagnetic voltage field of the galvanization process, chromium will again separate out at such protruberances and as the final product there will be present a rough mold inner surface possessing all of the previously described drawbacks.